London's Daily Mail reported that the lagoon is filled with one-celled organisms believed to be half-animal and half-plant called dinoflagellates that glow like fireflies in the water. An enzyme called luciferase made the organisms bioluminescent.

"We have been compiling data," Puerto Rico's secretary of the Department of Natural Resources Carmen Guerrero said. "There are a lot of factors that could be at play."

Fajardo Mayor Anibal Melendez told the AP he believed runoff from the construction of a nearby water and sewer treatment plant has caused the lagoon to go dark and has asked for the plant to be moved.

"We've never seen anything like that," Melendez said about the darkness of the lagoon.

Guerrero told the AP there could be more reasons than just the runoffs. She said rainstorms that generate heavy waves could be another possibility for affecting the lagoon's bioluminescence.

"It's important to give these experts room so they can do their job and help us understand what happened in the lagoon and why it has temporarily lost some of its brightness," Guerrero told the AP.

Alberto Lazaro, of Puerto Rico's water and sewer authority, told the AP that the water and sewer plant, set to be completed in 2016, was badly needed in the area because sewage is being dumped into the lagoon near ocean.